U.S. patent number 5,359,170 [Application Number 07/838,637] was granted by the patent office on 1994-10-25 for apparatus for bonding external leads of an integrated circuit.
This patent grant is currently assigned to AT&T Global Information Solutions Company. Invention is credited to Sy-Hwa Chen, Terry Craps, Johnston W. Peeples.
United States Patent |
5,359,170 |
Chen , et al. |
October 25, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus for bonding external leads of an integrated circuit
Abstract
A hot bar apparatus for effecting solder bonds for high density
electronic components has a diamond or diamond-like (i.e. a
generally diamond crystal lattice with graphite impurities) surface
that makes contact with the leads of the electronic component and
transmits the heat necessary to effect a solder bond. The diamond
or diamond-like surface has superior wear characteristics over any
other material, and it also is an electrical insulator to protect
the electronic components from spurious voltages and currents
generated by electrical currents that provide the heat for the hot
bar.
Inventors: |
Chen; Sy-Hwa (Columbia, SC),
Peeples; Johnston W. (Columbia, SC), Craps; Terry
(Lexington, SC) |
Assignee: |
AT&T Global Information
Solutions Company (Dayton, OH)
|
Family
ID: |
25277661 |
Appl.
No.: |
07/838,637 |
Filed: |
February 18, 1992 |
Current U.S.
Class: |
219/85.18;
219/85.16; 228/44.7 |
Current CPC
Class: |
H05K
3/3421 (20130101); B23K 3/047 (20130101); H05K
2201/10689 (20130101); Y02P 70/50 (20151101); B23K
2101/40 (20180801); Y02P 70/613 (20151101); H05K
2203/0195 (20130101) |
Current International
Class: |
B23K
3/04 (20060101); B23K 3/047 (20060101); H05K
3/34 (20060101); B23K 003/047 () |
Field of
Search: |
;219/85.16,85.18,228,229,230,233
;228/51,20,52,53,54,122,44.7,180.2,9,180.21,20.5 ;427/314,318 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0032437 |
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Jul 1981 |
|
EP |
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1-113172 |
|
May 1989 |
|
JP |
|
1-178365 |
|
Jul 1989 |
|
JP |
|
1-186269 |
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Jul 1989 |
|
JP |
|
2-224349 |
|
Sep 1990 |
|
JP |
|
3-33095 |
|
Feb 1991 |
|
JP |
|
3-198360 |
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Aug 1991 |
|
JP |
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3-261671 |
|
Nov 1991 |
|
JP |
|
Other References
"Diamond-Tipped and Other New Thermodes for Device Bonding", IEEE
Transactions on Components, Hybrids, and Manufacturing Technology,
vol. CHMT-2, No. 3, Sep. 1979, pp. 283-288. .
"Boron-Nitride Coated Thermode Blade for Surface-Mounted Technology
Interconnections", vol. 31, No. 3, Aug. 1988, pp. 98-99..
|
Primary Examiner: Evans; Geoffrey S.
Attorney, Agent or Firm: Martin; Paul W. Penrod; Jack R.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. An apparatus for soldering each external lead of an integrated
circuit to a respective conductor comprising:
a first thermode for converting a first electrical current into
heat to effect a first temperature thereof including first and
second adjacent strut members having first and second electrical
connectors at top ends of first and second strut member and having
a hot bar made of a refractory metal connected at lower ends of the
first and second strut members to form a first soldering end;
a second thermode for converting a second electrical current into
heat to effect a second temperature thereof including first and
second adjacent strut members having first and second electrical
connectors at top ends of the first and second strut members and
having a hot bar made of the refractory metal connected at lower
ends of the first and second strut members to form a first
soldering end facing the first soldering end of the first
thermode;
a first temperature sensor attached to the middle of the hot bar of
the first thermode for sensing said first temperature of said first
thermode;
a second temperature sensor attached to the middle of the hot bar
of the second thermode for sensing said second temperature of said
second thermode;
controller connected to said first and second temperature sensors
and said first and second thermodes to limit said first and second
electrical currents to maintain said first and second temperatures
at first and second predetermined levels and then to reduce them in
accordance with a temperature profile;
a first diamond coating covering a portion of said first thermode
that provides a first durable layer that said heat is transferred
through to solder bond a first external lead to its respective
conductor; and
a second diamond coating covering a portion of said second thermode
that provides a second durable layer that said heat is transferred
through to solder bond a second external lead to its respective
conductor;
said first and second diamond coatings being formed by a low
pressure deposition process;
said first and second diamond coatings being located in physical
contact with the first and second exterior leads while the first
and second temperatures are being maintained to melt solder between
the first and second exterior leads and their respective
conductors; and
said first and second diamond coatings being located in physical
contact with the first and second exterior leads while the first
and second temperatures are being reduced to allow the solder to
solidify and form bonds.
2. An apparatus for soldering each external lead of an integrated
circuit as set forth in claim 1 wherein said first and second
diamond coatings are of a single crystal type.
3. An apparatus for soldering each external lead of an integrated
circuit as set forth in claim 1 wherein said first and second
diamond coatings are of a poly-crystalline type.
Description
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for bonding
leads of an electrical device, and more particularly to a method
and apparatus for bonding external leads of an integrated circuit
or similar device to effect electrical and physical
connections.
A commonly known type of physically and electrically bonding an
electrical or electronic component to an electrical circuit is
referred to as soldering. In soldering, a molten metal, such as a
tin-lead alloy, is applied to a junction between a component and
the circuit it is to be bonded to. When the molten metal at this
junction solidifies by cooling, the component is fused and bonded
to the circuit.
There are numerous methods of melting and applying the solder to
the component. One method is to prepare a bath of molten solder and
then dip the leads to be soldered into this bath. A special version
of this method is called wave soldering and is used on through-hole
printed circuit boards. This is an excellent method, but it is
limited to the soldering of larger components with significant
lead-to-lead separation. Unfortunately for the more modern high
density components, e.g. lead pitches of 0.020 inches, solder bath
techniques not only solder the component lead to the desired
conductor, but also bridge over to make undesired connections with
adjacent leads and adjacent conductors.
Another method is to apply heat by means of a soldering iron to the
junction of a component lead, its connection conductor and some
solid solder in order to melt the solder and form the desired bond.
One such soldering iron is described in U.S. Pat. No. 2,501,616
entitled Soldering Iron Tip and issued to P. Robinson. This patent
explains that some of the usual problems of a soldering iron are
that the soldering surface degrades and changes character-istics
with use, with the result being poor and inconsistent solder
bonds.
An updated version of the soldering iron is called a hot bar. A hot
bar may have various geometries in order to solder all leads on at
least one side at once. Some hot bar machines solder all leads of
an integrated circuit in one operation. The hot bar is precisely
placed over the leads of an integrated circuit and then lowered
into contact with all of the leads at once. The temperature and
amount of heat transferred to the leads are carefully controlled to
obtain consistent results.
Unfortunately, the electrical currents that are passed through each
hot bar thermode to raise the temperature to the proper level
induces a local electric field of a level that can be damaging to
integrated circuitry.
The surface of the hot bar, like the soldering iron, also degrades
and changes with use. The soldering surface of the hot bar becomes
scratched, distorted and worn with use. Distortions and wear are
especially troublesome because even and consistent physical contact
is necessary for consistent solder bonds. Further, flux residues
and dirt will adhere and build up on scratches of the hot bar. This
build up not only causes inconsistent heating, but the dirt and
scratches also interact with the solder to cause solder bridges to
form between the integrated circuit leads, or solder to adhere to
both the integrated circuit lead and the hot bar during a cool down
cycle, thereby causing a tearing of the solder.
It is an object of the present invention to provide a hot bar
soldering surface that is immune to scratches and wear.
It is another object of the present invention to provide a hot bar
soldering surface that does not appreciably change its heat
transfer characteristics with use.
It is another object of the present invention to provide a hot bar
soldering surface that has a high thermal conductance and a low
electrical conductance.
SUMMARY OF THE INVENTION
Briefly stated, in accordance with one aspect of the invention the
foregoing objects are achieved by providing a hot bar that has a
soldering surface made of elemental carbon which is in a
crystalline form that is generally cubic, for example diamond and
diamond-like materials, the latter having graphite impurities in
the crystal lattice.
In accordance with another aspect of the invention, the foregoing
objects are achieved by providing a soldering device. The soldering
device includes a bar made of a refractory metal. The bar of
refractory metal has a layer of elemental carbon of a cubic
crystalline form, such as diamond or diamond-like materials,
covering it.
In accordance with a another aspect of the invention the foregoing
objects are achieved by providing an apparatus for soldering each
external lead of an integrated circuit to a respective conductor.
The apparatus includes a thermode for converting an electrical
current into heat to effect the temperature of the thermode. A
temperature sensor senses the temperature of the thermode and
communicates a signal to a control unit that the temperature sensor
is connected to. The control unit limits the flow of electrical
current according to the signal from the temperature sensor in
order to control the temperature of the thermode to predetermined
levels. The thermode has a coating of diamond or diamond-like
material covering a portion thereof to provide a durable layer that
heat is transferred through in order to solder bond each external
lead to its respective conductor.
In accordance with another aspect of the invention, the foregoing
objects are achieved by providing a method of bonding an exterior
lead of an electrical device to a conductor. The method includes
the steps of locating the exterior lead of the electrical device in
physical contact with the conductor, locating a diamond covered
surface in physical contact with the exterior lead of the
electrical device, transferring heat through the diamond covered
surface in the presence of a quantity of solder to cause the solder
to melt and flow onto the exterior lead of the electrical device
and the conductor, and reducing the heat transferred through the
diamond surface allowing the solder to solidify and form a bond
between the exterior lead and the conductor.
In accordance with another aspect of the invention, the foregoing
objects are achieved by providing a method of bonding an exterior
lead of an electrical device that has a quantity of solder
deposited thereon to a conductor. The method includes the steps of
locating the exterior lead of the electrical device in physical
contact with the conductor, locating a diamond covered surface in
physical contact with the exterior lead of the electrical device,
transferring heat through the diamond covered surface to melt the
quantity of solder to cause the solder to melt and flow onto the
exterior lead of the electrical device and the conductor, and
reducing the heat transferred through the diamond surface allowing
the solder to solidify and form a bond between the exterior lead
and the conductor.
In accordance with another aspect of the invention, there is
provided a method for manufacturing a soldering device. The method
of manufacture includes the steps of covering a bar with a layer of
elemental carbon of a cubic crystalline form of diamond or
diamond-like materials and attaching the bar to a body of the
soldering device such that the layer of elemental carbon of a cubic
crystalline form is the soldering surface for the soldering
device.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with the appended claims
particularly pointing out and distinctly claiming the subject
matter which is regarded as the invention, it is believed that the
invention will be better understood from the following description
taken in conjunction with the accompanying drawings in which:
FIG. 1 is a front view of a hot bar apparatus having two thermodes
for bonding external leads of a small outline integrated circuit to
conductors in accordance with the invention.
FIG. 2 is partially broken away side view of the hot bar apparatus
and small outline integrated circuit shown tin FIG. 1, in
accordance with the invention.
FIG. 3 is a perspective view of a hot bar with a coating of diamond
or diamond-like material before it has been attached to a thermode
in accordance with the invention.
FIG. 4 is a perspective view of a hot bar with a layer of diamond
or diamond-like material attached thereon before the hot bar has
been attached to a thermode in accordance with the invention.
DETAILED DESCRIPTION
Referring now to FIG. 1, a bonding apparatus 10 is shown. The
bonding apparatus 10 is connected to larger machine, such as a Hot
Bar Bonder by Universal Instruments or a similar device. Such a
device has a carrier (not shown) to which the bonding apparatus 10
is attached. The bonding apparatus 10 has two opposing thermodes
12, 14. The two thermode example was selected for simplicity of
description, but the invention is not limited to two thermodes, in
fact an apparatus with four thermodes is contemplated for
integrated circuits with external leads on four sides.
The location of the carrier and thermodes 12, 14 is controlled, for
example by robotic control, in a horizontal plane above an
electronic module 18 until it is over the integrated circuit 20 and
the conductors 22, 24 that are attached to the module 18. Once the
thermodes 12, 14 are located over the integrated circuit 20 and the
integrated circuit's respective conductors 24 on the module 18, the
thermodes 12, 14 are lowered until physical contact is made between
the thermodes 12, 14 and the external leads 22 of the integrated
circuit 20.
Typically a coat of a bonding material 26, such as tin/lead solder,
is applied beforehand to the external leads 22 of the integrated
circuit 20, the conductors 24, or both. To effect a bond, the
thermodes 12, 14 after they are lowered onto the external leads 22
are heated such that the bonding material 26 melts and flows around
and between the external leads 22 and the conductors 24. After the
bonding material 26 has melted and allowed to flow, it is allowed
to cool until it solidifies, in order to form bonds that are
physically strong and electrically conductive. Bonds similar to the
ones produced by the thermodes 12, 14 are well known in the
art.
Referring now to FIG. 2, the bonding apparatus 10 will be described
further. Since The bonding apparatus 10 has two thermodes 12, 14
that are substantially mirror images of each other, only the
thermode 12 will be described. Those skilled in the art will
understand the invention from the description of the thermode
12.
The thermode 12 has two mounting brackets 32, 33 that are insulated
from each other to prevent electrical short circuits. Strut 30 is
connected to and projects from bracket 32. Similarly, strut 31 is
connected to and projects from bracket 33. Each of the brackets 32,
33 has an electrical connection point 34, 35. These connection
points are for the connection of conductors (not shown) that carry
heat producing electrical current. The struts 30, 31 and the
brackets 32, 33 are made of materials that have good electrical
conductances, such as a copper alloys, and also have good
rigidities so that they keep their shape during repeated use.
The struts 30, 31 are connected at their lower ends to a hot bar
38, which is also of an electrically conductive material. The hot
bar 38 is designed such that it heats up as an electrical current
is passed through it. Since the hot bar 38 is also the portion of
the thermode 12 that make physical contact with the integrated
circuit conductors 24, it is important that the hot bar 38 retain
its shape during use. For these reasons, the hot bar 38 is made of
a refractory metal such as titanium.
To provide heat, electrical current is conducted through struts 30,
31, connection points 34, 35 and the hot bar 38. A thermocouple 40
is attached to the middle of the hot bar 38. The thermocouple 40
outputs an electrical signal that is a known function of the
temperature of the bar. This signal is used to sense the
temperature of the hot bar 38 so the heat producing electrical
current through the hot bar 38 may be controlled to provide the
desired temperature during each soldering operation. This
temperature and time function is sometimes called a temperature
profile.
Referring now to FIGS. 2 and 3, one embodiment of the invention
will be described. FIG. 3 shows a hot bar 38 that has a diamond or
diamond-like coating 42 on it. This diamond or diamond-like coating
may be effected by a low pressure deposition process or a similar
process. The coating can be deposited either as a single crystal
coating or as a poly-crystalline coating.
The coating 42 may be deposited on the hot bar 38 with openings in
the coating to allow for brazing of the hot bar 38 to the struts
30, 31, or the hot bar 38 may be fastened to the struts 30, 31 by
brazing or similar unitary forming process and then the coating
deposited on the hot bar 38. In either case, the temperature of the
hot bar 38 can be increased during the depositing process in order
to prestress the hot bar 38 by thermal expansion thereof. This
prestressing prevents the coating from fracturing from thermal
expansion of the hot bar 38 when the heat producing current is
applied.
Referring now to FIGS. 2 and 4, a second embodiment of the
invention will be described. FIG. 4 shows a hot bar 38A that is of
similar material and has similar dimensions as the hot bar 38 shown
in FIG. 3. The hot bar 38A has a layer 44 of diamond or
diamond-like material attached to one side thereof. The layer 44
may be a slab off of a naturally occurring diamond, or it could be
a slab off of a diamond or diamond-like crystal formed by a high
temperature and high pressure manufacturing process. The layer 44
can be either single crystal type or poly-crystalline type.
The layer 44 is fastened to the hot bar 38A by brazing, or similar
unitary forming process. The hot bar 38A is attached by brazing or
some similar unitary forming operation to the struts 30, 31 either
before or after the fastening of the layer 44 to the hot bar 38A.
Since brazing and similar processes heat up the hot bar 38A and the
layer 44, some prestressing by thermal expansion of the hot bar 38A
and the layer 44 is involved in the manufacturing process of this
embodiment of the invention.
Thus, it will now be understood that there has been disclosed a new
and novel bonding apparatus which includes thermodes having diamond
covered soldering surfaces. Although the present invention may be
used for almost any soldering operation, it is very advantageous
when used to solder external leads of surface mount components.
While the invention has been particularly illustrated and described
with reference to preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in
form, details, and applications may be made therein. For example,
the present invention could be practiced on soldering heads that
solder components that have leads on one, three or four sides
instead of two. It is accordingly intended that the appended claims
shall cover all such changes in form, details and applications
which do not depart from the true spirit and scope of the
invention.
* * * * *